1. Technical Field
The present invention relates in general to CAD/CAM applications in data processing systems and in particular to methods and systems for display of CAD/CAM drawings. Still more particularly, the present invention relates to a method and system for correction of stippled lines utilized in CAD/CAM drawings and display accuracy of the line.
2. Description of the Related Art
Data processing Systems that are primarily employed for graphics processes are used in many different areas of industry, business, education, home and government. Graphics applications for these systems are growing rapidly and include interactive planning, office automation, electronic publishing, animation and computer-aided design. The applications are developed utilizing various software "tools" that best work with the available data processing system hardware.
Computer Aided Design/Computer Aided Manufacturing ("CAD/CAM") is utilized to assist in preparing drawings to assist in design and building models of potential products. CAD/CAM plays a great part in providing drawings and assisting in manufacturing components of electrical, mechanical and electromechanical items such as automobile engines, draw works, oil well drilling rigs, high rise buildings, micro-processor devices and including airplanes, ships and space vehicles. Often the designs are implemented via drafting or blueprints. However, the designs are often utilized, interactively on a computer display, to determine possible physical properties of design elements by subjecting an on-line display model to various virtual stresses. By simulating actual conditions through virtual testing on a computer terminal, a design may be pre-tested without having to actually build the final product. It is important that the CAD design be accurate and rendered in sufficient detail to provide reliable results of the on-line testing.
CAD/CAM applications and draftspersons use stippled ("styled") lines to indicate specific drawing details. Stippled lines are lines comprised of different combinations of dashes and dots where each combination illustrates a particular type of view in a drawing. For example, in a drawing, one dashed line may indicate a hidden edge within a solid. Another dashed line may indicate a centerline of the same object. If the dashes are distorted, the lines may be confused. Therefore, it is important that the line stipple ("style") be easily distinguishable from other line styles.
On computer graphics systems, a line style is typically defined by a sequence of ones and zeroes. The line style is displayed on a video screen as a pattern of light ("on") and dark ("off") pixels in a rectangular array ("raster display") and identified by the number of on and off pixels (for example, 24 pixels on, 8 pixels off). Line rasterization is performed utilizing a Bresenham (developer of the technique) line drawing technique in which the line is rasterized with respect to its major axis (the axis in which the distance between its endpoints is largest). For each step along the major axis, a pixel is rendered (light, dark and sometimes shaded) and the line style pointer (a marker indicating the current position within the line style pattern) is incremented to point to the next entry in the line style pattern. The rendered pixel is drawn ("on" or shaded) if the current line style pointer points to a one in the pattern and not drawn ("off") if it points to a zero.
Since sequencing is done with respect to the major axis (either X or Y), lines drawn on the diagonal (dX=dY) result in a line style that appears to be 41% longer than the same line drawn horizontal or vertical. Any deviation from true horizontal or true vertical results in a line depicted with increased stipple length.
Current industry standard graphics APIs like OpenGL.TM., a graphics application interface of Silicon Graphics, Inc. of San Jose, Calif., are specified such that anti-aliased, stippled lines should be slope corrected. A stippled line on a display is considered to be a sequence of contiguous rectangles. Each rectangle is a size, in width and length, which is equal to one pixel and each rectangle is centered on the line segment. The rectangles referred to are pixels. To date, most vendors have not implemented slope correction of nominal width lines because of the expense in doing so.
In the past, several techniques have been utilized to slope correct a stippled line. One method of slope correction of the line style considered the line style pointer to be a fixed point number with one fractional bit.
FIG. 5, a flow diagram illustrating a method for slope correcting line stipples, illustrates this method.
The process begins at step 500, which depicts the application determining the originating and terminating vertex of the line to be slope corrected. The process proceeds to step 502, which illustrates determining the major and minor axis of the stipple segments within the line, where major axis is the axis with the largest change. The process then passes to step 504, which depicts the application beginning the procedure to slope correct the line. The process passes next to step 506, which illustrates adding 1.0 to the line style pointer at the first stipple segment in the line. For every step along the major axis, 1.0 is added to the line style pointer. The process continues to step 508, which depicts adding 0.5 to the style pointer and for every step along the minor axis, 0.5 is added to a line style counter. The line style count is utilized to step through the line style pattern as stippled lines are rasterized. The process next passes to step 510, which illustrates repeating the two preceding steps, 506 and 508, until the terminating vertex of the line is reached.
If this procedure is not applied to a line, the length of the individual stipples will appear longer than the true length. This correction technique, albeit very simple to implement, results in line stipples that can still be in error by over 10%.
Another known method uses line style assist logic that consists of a count down counter and a shrink/expand control bit. For every step along the major axis, the line style counter is incremented and the count down counter decremented. If the count down counter reaches zero, the counter is reloaded and the line style counter is incremented if shrink control is selected or decremented if expand control is selected. The accurate line style assist logic suffers from significant accuracy problems because not enough precision exists where it is needed the most (when the count down counter is small).
It would be desirable, therefore, to provide a method that would utilize presently existing hardware to provide accurate displays of line stipple in lines off the vertical or horizontal planes.